Although the present invention may be used advantageously on many different types of blowers, regardless of the manner of input drive to the blower, the present invention is especially adapted for use with a Roots-type rotary blower that is driven by an internal combustion engine. In a typical internal combustion engine used commercially for on-highway vehicles, the torque output of the engine is not perfectly smooth and constant, but instead, is generated in response to a series of individual, discrete combustion cycles.
A typical Roots-type blower transfers volumes of air from the inlet port to the outlet port, whereas a screw compressor actually achieves internal compression of the air before delivering it to the outlet port. However, for purposes of the present invention, the blower, or compressor, generally includes a pair of rotors, which must be timed in relationship to each other, and therefore, are driven by meshed timing gears. As is now well known to those skilled in the blower art, the timing gears are potentially subject to conditions such as gear rattle and bounce.
Rotary blowers of the type to which the present invention relates (e.g., either Roots-type or screw compressor type) are also referred to as “superchargers,” because they are used to effectively supercharge the intake side of the engine. Typically, the input to an engine supercharger is a pulley and belt drive arrangement that is configured and sized such that, at any given engine speed, the amount of air being transferred into the intake manifold is greater than the instantaneous displacement of the engine, thus increasing the air pressure within the intake manifold, and increasing the power density of the engine.
Rotary blowers of either the Roots-type or the screw compressor type are characterized by the potential to generate noise. For example, Roots-type blower noise may be classified as either of two types. The first is solid borne noise caused by rotation of timing gears and rotor shaft bearings subjected to fluctuating loads (the periodic firing pulses of the engine). The noise, which may be produced by the meshed teeth of the timing gears during unloaded (non-supercharging), low-speed operation is also referred to as “gear rattle.” The second type of noise is fluid borne noise caused by fluid flow characteristics, such as rapid changes in the velocity of the fluid (i.e., the air being transferred by the supercharger). The present invention is concerned primarily with the solid borne noise caused by the meshing of the timing gears.
To minimize solid borne noise, torsion damping mechanisms (e.g., “isolators”) have been developed, which can minimize the “bounce” of the timing gears during times of relatively low speed operation, when the blower rotors are not “under load.” Such torsion damping mechanisms are also referred to as “isolators” because part of their function is to isolate the timing gears from the speed and torque fluctuations of the input to the supercharger. A torsion damping mechanism or torsional isolator may have the opportunity to create a noise referred to as clunk. Clunk noise is generated when the negative input torque exceeds the isolator's negative torque capacity. The clunk noise includes the noise generated by impacts within the mechanical components of the isolator and the impact of the timing gear teeth to each other.